Trees & BST

This is a premium interview-preparation course on the data structure that shows up in more interviews than any other: the **binary tree**, and its ordered cousin the **binary search tree**. Trees reward a single mental shift — stop thinking about the whole structure and start thinking about one node, its left subtree, and its right subtree. Almost every problem here is solved by asking what a node needs from its children (a bottom-up **depth-first** pass) or what it needs to pass down (a top-down pass), or by sweeping the tree one level at a time (**breadth-first**). You will start with the vocabulary — nodes, height, depth, balance, and the four traversal orders — then work through curated problems grouped by the patterns interviewers test: DFS traversals, BFS level-order, the BST ordering invariant, recursive subtree aggregation, rebuilding a tree from its traversals, and advanced problems that combine these ideas. Every lesson visualises the recursion or the level sweep on a concrete tree before showing a clean Java 17 solution.